It has been the practice in the past in certain instances to use environmental air ducts and plenums to provide a passageway for electrical cables. The flame retardant characteristics of electrical cables conveyed through such ducts and plenums is necessarily extremely critical particularly with respect to flame propagation when exposed to a flame.
One test for evaluating flame propagation of electrical cables for use in ducts and plenums (commonly called "plenum cables") is Underwriters Laboratories Inc., Tunnel Flame Test designated UL 910. The UL 910 Test (also known as the "Steiner"Test) is used as a standard by industry for determining the suitability of particular cables for use in ducts and plenums. The Test, which is described in an Underwriters Laboratories publication titled "Test Method for Fire and Smoke Characteristics of Cables", copyright 1981, 1982, the disclosure of which is incorporated herein by reference, generally involves laying 24 ft. lengths of the cable to be tested adjacent each other on supporting racks within a prescribed test chamber and, after following a prescribed calibration procedure, exposing one end of the cables to an approximate 300,000 BTu methane flame for a prescribed period of time while measuring flame propagation and smoke generation in terms of specific optical density in accordance with the test procedures. Underwriters Laboratories has established that electrical cables suitable for use in ducts and plenums must exhibit a smoke generation characteristic in terms of peak optical density of 0.5 or less and a maximum flame spread of 5 feet or less.
The electrical cable of the present invention is able to meet or exceed the flame spread requirements of the UL 910 Test without having to incorporate the amount of fluorocarbon polymers heretofore thought necessary in the past and, although certain embodiments may not meet the smoke generation requirements of the UL 910 Test, enable their use in many applications in which the prevention of flame propagation is the most important criterion.
Up until the time of the present invention, plenum cable has exclusively utilized fluorocarbon polymeric material such as fluorinated ethylene propylene as a means of providing conductor insulation and jacketing able to meet or exceed the requirements of the UL 910 Test. Such fluorocarbon materials, however, are expensive, and contain substantial amounts of fluorine which in a burning environment are apt to release a complex series of potentially toxic or corrosive gases. Often such cables additionally utilize polymers that contain other halogens such as chlorine in combination with fluorocarbon polymers which in a burning environment and particularly in combination with fluorine are also apt to release a complex series of potentially toxic or corrosive gases.
In view of the above, a need exists for an electrical cable that is able to meet or exceed the flame spread requirements of the UL 910 Test which utilize one or more polymers having a minimum amount of fluorine and other halogens.